Field of the Invention
The present invention relates to a bladed rotor wheel for a turbine engine, and more particularly to a fan wheel of a turbine engine such as an airplane turboprop or turbojet.
Description of the Related Art
Conventionally, a fan wheel comprises a disk carrying blades at its outer periphery, the blades having roots that are engaged in substantially axial slots formed in the outer periphery of the disk. The fan blades are held radially on the disk by co-operation between the shape of their roots and the shape of the slots in the disk, the blade roots being of the dovetail type, for example. Inter-blade platforms are mounted on the disk between the fan blades or they constitute an integral portion of the disk. Blades of this type are described by way of example in document FR 2 881 174 in the name of the Applicant. The slots may be straight or curvilinear and the contact surfaces between the blade roots and the inside walls of the slots are referred to as “bearing” surfaces.
The blades are mounted with clearance between their roots and the walls of the slots. In order to reduce this clearance, proposals have been made to mount resilient spacers between the bottoms of the slots and the blade roots, so that the blade roots are held pressed radially against the bearing surfaces of the disk.
The spacers need to be of high precision in order to ensure that the blades are held without being completely blocked in a position that would prevent the blade roots form bearing normally against the bearing surfaces of the slots.
That type of rotor wheel presents the drawbacks set out below.
The rotation of the rotor wheel while the engine is in operation causes large centrifugal forces to be applied to the blades, thereby holding them in their “operating” positions. By way of example, a fan blade weighing about 4.5 kilograms (kg) may be subjected to a force that is greater than 60 (metric) tonnes (t).
For a fan wheel in particular, after the blades have been assembled on the wheel, any unbalance that appears during rotation of the wheel is measured. If this unbalance is greater than a determined threshold, certain blades are disassembled and interchanged in order to reduce the unbalance.
In the absence of a spacer, or when the spacers are insufficiently prestressed, every time the rotor wheel is stopped, it occupies an angular position that is different, and the blades can change position under the effect of their own weight. Under such circumstances, since the positions of the blades, and thus the unbalance of the rotor wheel, are different from one start to another, it is difficult to measure and reduce any unbalance.
In addition, the small-amplitude movements of the blades under high loading that take place each time the engine is started cause the inside surfaces of the assembly slots for the blade roots to be worn. This is known as “fretting” wear, and it can prevent the integrity of the disk and of the blades being inspected by means of eddy currents, which in turn can lead to a disk and/or blades being scrapped, even if they are in fact still usable.
Furthermore, when fan blades are made of composite material, it is not possible to mount the spacers while subjecting the blade roots to high stress, since the roots cannot withstand such stresses.
Patent application FR 11/54301 in the name of the Applicant and not yet published, describes a bladed rotor wheel for a turbine engine, the rotor wheel comprising a disk and spacers mounted between the blade roots and the bottoms of the slots in the disk. The spacers are of the type that is bistable in position and each of them can occupy a first stable position for assembly or disassembly in which they do not exert force on the blade roots, and a second stable position in which they exert a radial force on the blade roots in order to hold the blades stationary and stabilize them in a final position.
In that way, it is possible, while these spacers are in a first position, to assemble them together with the blades in the slots of the disk, and then to rotate the disk at a speed that is sufficient for the spacers and the blades to take up their final positions, with each spacer becoming deformed so as to pass from its first position to its second position, the blades subsequently being held in their final positions by the spacers, even when the disk is rotating slowly or stopped.
For rotor wheels of a low pressure or a high pressure compressor of a turbine engine, first rotation of the wheel is performed at a speed lying in the range 2500 revolutions per minute (rpm) to 4000 rpm, so as to cause the spacers to deform. Simultaneously, the tips of the blades are trued at high speed. Thereafter, the rotor wheel is balanced at low speed without disassembling the blades.
The use of shape memory spacers is also known from document DE 3 236 021.
For blades of large size, such as fan blades in particular, the solution proposed in document FR 11/54301 presents the drawbacks set out below.
The fan wheel is surrounded by a fan casing having its inside surface provided with an abradable material, the radially outer ends of the blades coming flush with the inside surface of the casing.
Given the size of the blades, the radially outer ends of the blades can move considerably on first rotation of the wheel. Those ends then degrade the abradable material of the casing significantly.
For fan wheels in particular, it is not envisaged that the wheel might be set into rotation without its casing.
Furthermore, in the event of a bird or an article of significant size being ingested, the fan blades are subjected to violent impacts that cause them to tilt. These impacts must be damped and the tilting of the blades must be limited.
Unfortunately, under certain circumstances, the prior art bistable spacer is found to be incapable of damping the impact energy or of limiting the movement of the blade.
Finally, it is possible to use bladed disks made as single blocks, in which the blades and the disks are made in a single block by machining.
Nevertheless, that solution is not applicable to all rotors, in particular because of thermal stresses, of mechanical stresses, and of weight constraints, and costs are very high both for manufacture and for maintenance.